The present invention relates to a monitoring device for sensing and controlling the operational status of low current devices and, more particularly, to a status relay incorporating a current switch in a sensing circuit.
Many industrial environments use large numbers of electrical devices that, individually, draw very limited amounts of current, often only a small fraction of an amp. However, these devices, such as small fan motors, can be very important to the successful completion of a process that may involve costly or hazardous equipment or materials. Typically, the operation of the various remotely located devices powering complex industrial processes and machinery is monitored and controlled from a central control. A protection device or status relay, including a fault detection mechanism, monitors the operational status of the remote device and transmits a signal to the central control if the operation of the device changes in a significant manner. The fault detection signal may be displayed on a control panel to inform a human operator permitting the operator to respond to changes in the operation of the device. The fault detection signal is also commonly input to a controller that utilizes a logic process to determine the effect of the status change on the process, machinery, or system and provide an appropriate control signal to control the device.
Electrical circuit fault detection is commonly provided by a current sensor that is electrically coupled to a cable supplying power to an electrical device or load. The current sensor is electrically connected to the remotely located central control to provide a signal to the control that is representative of the status of the current within the cable. A relay or similar device may be interconnected between the remotely located control and the electrical device to receive a control signal from the control and, in response, selectively enable or disable power to the load or power to a signal terminal of a local device controller. The relay may function as a starter for the device if the power rating of the relay is appropriate, but may, if desired, be interconnected to a separate starter controlling power to the load.
Holce et al., U.S. Pat. No. 5,808,846, incorporated herein by reference, disclose a protection device comprising a combination current sensor and relay for monitoring current in a cable supplying power to a device and controlling the device in response to a signal from a remotely located control panel. The protection device includes a sensing transformer comprising a wire wound core. A changing current in the power cable produces a changing electromagnetic field around the cable which, in turn, induces a magnetic flux in the core of the sensing transformer. The magnetic flux in the core induces a voltage in the wire windings that is representative of the current flowing in the power cable. Thus, the power cable is the primary winding and the wire winding is the secondary winding of the sensing transformer. The wire winding is electrically connected to an input circuit that converts the voltage signal received from the secondary winding of the sensing transformer to an output signal representative of the current flowing in the power cable. The output signal is transmitted to a control panel and analyzed to determine if the device is to be disabled or enabled. The control panel transmits an appropriate signal to a switch circuit, typically comprising a triac or relay, which responds to the signal from the control panel by shorting or isolating electrical terminals in series with the controlled device.
While the protection device disclosed by Holce et al. is compact and easy to install, it functions best with devices that draw substantial current. The current sensing input circuit is powered by energy sourced from the power cable through the wire winding of the sensing transformer. If the power cable current level is low, the energy required to power the passive input circuit is sufficient to make measuring of the current to the load unreliable. Further, a minimum current of approximately 0.25 amps is required to generate sufficient flux to power the input circuit. The current draw of many electrical devices, including fractional horsepower motors, is insufficient for reliable sensing with this type of sensing circuitry. What is desired, therefore, is a protection device for electrical devices or loads that operate at low current levels.